Most sponge rubbers used for automobiles and buildings are composed of an ethylene-.alpha.-olefin copolymer rubber which is superior in heat resistance, weatherability, workability, and production cost. Such a rubber is extensively popularized particularly in use for door seals, trunk seals, and window seals of automobiles as an indispensable material.
However, as the result of improvements of the performance of automobiles, the performances standard required for sponge rubbers have become higher, so that rubbers produced by the conventional techniques are not sufficient to meet all the required stringent standards.
Specifically, the problems of invasion, into a car interior, of engine noise, air-friction noise around doors, and squeaking noise of tires, leaking of rain, and other problems at high speed driving are dependent greatly upon sealing around the doors; and, therefore, the requirements for sealing performance of sponge rubbers are becoming more severe.
The door sealing sponge rubber, which is kept compressed for a long time when the doors are closed as a sealing material between a door and a car body, ideally should exhibit reduced settling under compression. The settling due to compression is generally expressed quantitatively in terms of compression set. Sealing sponge rubbers are desired to exhibit a smaller value of this compression set.
In addition to the settling, the vibration of doors at high speed driving must be taken into account. Since the vibration frequency of the door will increase with the increase in driving speed of an automobile, sealing rubbers sponge should be capable of following up sufficiently the vibration of the door.
If the sealing sponge rubbers cannot follow up the vibration of the door, a gap is formed between the door and the car body, causing the invasion of the above-mentioned various noises into the car interior. Such follow-up ability can be quantified generally by measuring hysteresis loss. The sponge rubber is desired to exhibit smaller hysteresis losses.
Furthermore, the doors should be able to be opened and closed smoothly over a broad temperature range from a high temperature to a low temperature, so the sponge rubber must have sufficient softness over a broad temperature range.
In addition to he above-mentioned requirements performance, the sponge rubber is naturally required to have good working characteristics such as kneading processability, extrusion processability, and shape-retaining ability.
To meet such requirements directed to the sponge rubber, there is, for example, known an ethylene-.alpha.-olefin copolymer having an ethylene unit content of from 55 to 73 mole % and a Mooney viscosity (ML.sub.1+4 121.degree. C.) of from 65 to 120 as proposed in JP-B-58-57450. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
In this proposed technique, however, the sponge rubber is unsatisfactory in compression set, hysteresis loss, and softness, even though it is satisfactory in green strength and shape-retention.